IDENTIFICATION OF GRAMINICIDE-RESISTANT JOHNSONGRASS (SORGHUM-HALEPENSE)

Resistance to fluazifop-P and quizalofop-P (aryloxyphenoxypropionates) and sethoxydim (cyclohexanedione) was identified in 2 populations of johnsongrass in both field and greenhouse studies. The cropping histor y (1983-1991) of the sites indicated 1 or more annual applications of a graminicide (primarily fluazifop-P) since the early 1980s. Under fie ld conditions, control of resistant seedling and rhizome johnsongrass (R91F) with fluazifop-P quizalofop-P fenoxaprop-ethyl, and sethoxy-dim was less than 35%. Clethodim provided up to 80% control of R91F. Unde r greenhouse conditions, ratios (R/S) of the I-50 values (amount of he rbicide required to inhibit plant growth by 50%) of resistant (2 sites : R91F and R91S) to susceptible (S91H) seedling (20-30-cm height) plan ts were > 388 (fluazifop-P), > 15 (quizalofop-P), and from 2.3 (R91S) to 3.4 (R91F) (both sechoxydim). For rhizome (30-45 cm height) plants, the R/S ratios were > 388 (fluazifop-P), > 16 (quizalofop-P), and 2.8 (R91S) to 8.5 (R91F) (both sethoxydim). Labeled rates (in kg ai ha(-1 )) of fluazifop-P (0.10 and 0.21), quizalofop-P (0.039 and 0.08), and sethoxydim (0.21 and 0.21) were applied on seedling and rhizome plants , respectively, and resulted in little or no control of resistant john songrass. Greenhouse studies indicated registered rates of clethodim ( 0.10 and 0.14 kg ai ha(-1) for seedling and rhizome plants, respective ly) effectively controlled the resistant populations, but tolerance wa s measured for both seedling and rhizome plants at sublethal doses (do wn to 0.007 and 0.009 kg ai ha(-1), respectively), with I-50 ratios ra nging from 1.5 (R91S) to 2.1 (R91F) for seedling plants and 4.5 (R91S) to 4.8 (R91F) for rhizome plants. Control of resistant seedling and r hizome johnsongrass under field conditions was adequate with glyphosat e at 0.84, glufosinate at 0.84, and sulfosate at 0.84 kg ai ha(-1), in dicating no cross-resistance.